1. Field of the Invention
The present invention relates to noise reduction technology for controlling exhaust noise in internal combustion engines and noise in ducts of air delivery systems, and more particularly, to an apparatus and method for controlling noise using bypass conduits or ducts and dual mufflers.
2. Description of the Prior Art
The most common method for controlling exhaust noise from an internal combustion engine is to use a muffler. For automobile engines, revolution speed varies greatly and the major frequency components of exhaust noise vary according to the revolution speed. It is thus very difficult to design a muffler so that it operates effectively over a wide range of revolution speeds. In general, conventional mufflers are designed so that exhaust gas passes through a complex pathway which essentially causes flow resistance for the exhaust gas, thereby increasing back pressure which hampers smooth exhaust of the gas from the engine and eventually lowers engine efficiency. In fact motorcyclists sometimes intentionally cut off the connection between the engine and the muffler to enjoy the resulting exorbitant noise and increased power of the engine. The problem is not an easy one to solve as reduction of exhaust noise has a direct correlation to increased back pressure and reduction in engine power.
Recent research on noise control involve studies on active control of exhaust noise to overcome the above-described disadvantages of passive noise control methods in an effort to increase engine efficiency and minimize back pressure. These active control studies have also been applied to noise reduction for ducts of air delivering systems such as air conditioners.
According to one example of a proposed active control system for exhaust noise, a noise measuring microphone is installed in an exhaust pipe. The noise registered on the microphone is transmitted to a controller which transmits an output signal via a control means to a speaker which is installed closer to an outlet than the microphone. Controlled noise, having the same magnitude as that of the original noise but at a phase opposite to that of the original noise, is generated through the speaker, and both of the original noise and the controlled noise are allowed to interfere with each other so that any emitted sound from the exhaust is reduced.
Although such active noise control methods might work in a laboratory, they have yet to be commercially applied to an actual vehicle because of the following problems which need to be solved.
(1) It is necessary to provide a speaker having very large output power for controlling large exhaust noise from a vehicle. Further, the size of the speaker must be increased to generate the required low frequency sound. Thus, it is difficult to manufacture a small and lightweight speaker having a high output power which are the requirements for installation onto areas where exhaust noise are generated on a vehicle. It is economically impractical to manufacture such a speaker.
(2) When applied to a vehicle, there is insufficient space for installing such a speaker which must have substantial size and weight. The lower structures of existing vehicles must be altered substanially and it is difficult to manufacture and maintain such a vehicle.
(3) Vehicular exhaust systems are connected by resilient members such as rubber rings to reduce vibration transmitted to the chassis. Attachment of a heavyweight speaker to an exhaust system will increase vibration due to extra weight and the exhaust system along with the speaker will deteriorate at a quicker rate.
(4) It will be extremely difficult to ensure durability of microphones and a speaker exposed to hot oxidizing exhaust gas.
(5) Controlled noise emitted from the speaker will flow backward up the tubing through diffraction and reflection, and will be measured together with the reference noise by the microphone positioned upstream of the exhaust pipe. Thus, the system will have to be very complex in design and expensive. Few automakers or consumers will be willing to pay for such non essentials.
Other solutions have been proposed including a method for actively controlling exhaust gas from an internal combustion engine or the duct of an air conduit, by using a bypass pipe such as shown in Korean Patent Publication No. 1995-2473 (FIG. 1). In such an apparatus, a U-shaped bypass pipe, the length of which can be varied, is attached to a main exhaust pipe. Such pipes are provided with a bypass region by which the main exhaust passage of the main pipe is bypassed and then the bypass passage through the bypass pipe is reintegrated with the main exhaust passage. The bypass region is comprised of a bypass pipe of variable length, composed of an outer and inner cylindrical portions which telescopically engage each other, and a fixed pipe of fixed length. The length of the bypass pipe can be changed by an actuator having an actuating rod which can be controlled by signals based on the noise collected from microphones.
With such a construction, the phase difference between the main noise components of the exhaust gas passing through the fixed pipe and the controlled noise components of the exhaust gas passing through the bypass pipe is adjusted 180 degrees. Consequently, the main noise components are eliminated by the controlled noise. In this case, the aforementioned problems can be avoided since the noise itself is used as a controlled noise without need of additional controlled noise sources such as a speaker However, the above method is capable of eliminating only the main noise component and their odd harmonics. Thus, it is impossible to control even harmonics of the main noise components generated from the engine together with eliminating their odd harmonics, and the method cannot provide a measure for controlling a broad band noise. In addition, when the engine operates at a low revolution such as in idling, the frequency of the main noise component is low, and its wavelength is large. In order to produce such a large phase difference between the noise having a large wavelength and the controlled noise, the length difference between the bypass pipe and the fixed pipe must be large. Consequently, the length of the bypass pipe is too long making its application for such purposes impractical.
Therefore, an object of the present invention is to solve the aforementioned problems, and to provide an apparatus and a method for controlling exhaust noise from an internal combustion engine by eliminating the main noise component of exhaust gas as well as their odd and even harmonics, which enhance engine efficiency by reducing back pressure of the exhaust gas, and eliminates broad band noise at high speed operation.
Another object of the invention is to provide an apparatus and a method for controlling noise in ducts of air conditioning systems which are capable of deleting the main noise component generated in the ducts as well as their odd and even harmonics.
The above objects can be accomplished by providing an active exhaust noise control apparatus comprising a main exhaust pipe, a first bypass pipe having a length which is variable and connected to said main exhaust pipe at both ends thereof so that a first bypass section is defined in the passage of the main exhaust pipe, a second bypass pipe having a length which is variable and connected to said main exhaust pipe at both ends thereof so that a second bypass section is defined in the passage of the main exhaust pipe, first and second actuators being actuated so as to vary the length of said first and second bypass pipes, respectively, and a controller for controlling said actuators. The active exhaust noise control apparatus of the present inventions may be provided with a lower back pressure muffler and a higher back pressure muffler bifurcated downstream of the main exhaust pipe, and a valve for selectively communicating the main exhaust pipe with the two mufflers, and wherein said valve is controlled by the controller.
The above objects can also be accomplished by providing a method of controlling exhaust noise in an engine by using the above apparatus, wherein at high speed operation of the engine, noise control is carried out by the steps of analyzing the main noise component C in the controller, removing the main noise component C and the noise components having frequencies of two times and odd time, such as three times or five times of the frequency of the main noise component by actuating the actuators and then adjusting the lengths of the bypass pipes, and smoothly discharging the exhaust gas by actuating the valve and thus passing the noise of the remaining wide range band through the low back pressure muffler, and when of starting the engine or driving at low speed, said noise control is carried out by actuating the valve and then passing the exhaust gas through the high back pressure muffler.
The above objects can also be accomplished by providing a noise control apparatus for controlling noise inside a duct of an air delivering system. Such apparatus is adapted for a main air delivering duct, and comprises a first bypass duct of which both ends are connected to said air delivering duct so that a first bypass section is defined in the passage of the air delivering duct, a second bypass duct of which both ends are connected to said main air delivering duct so that a second bypass section is defined in the passage of the main air delivering duct, the length of the first bypass duct is selected such that the lengths of the two air delivering passages passing through the two connection points of the main air delivering duct and the bypass duct differs from each other by a half wavelength of the main noise component occurring in the gas delivering system, and the length of the second bypass duct is selected so that the lengths of the two air delivering passages passing through the two connection points of the main air delivering duct and the bypass duct differs from each other by a half wavelength of components having a frequency of two times the frequency of the main noise component occurring in the air delivering system.